1999ApJ...512..601A

We present results from a long-term program to quantify the range of behavior of the centimeter-wavelength total flux and linear polarization variability properties of a sample of 41 radio-loud BL Lac objects using weekly to trimonthly observations with the University of Michigan 26 m telescope operating at 14.5, 8.0, and 4.8 GHz; these observations are used to identify class-dependent differences between these BL Lacs and QSOs in the Pearson-Readhead sample. As a group, the BL Lacs are found to be more highly variable in total flux density than the QSOs. These changes are often nearly simultaneous and of comparable amplitude at 14.5 and 4.8 GHz, which contrasts with the behavior in the QSOs and supports the existence of class-dependent differences in opacity within the parsec-scale jet flows. Structure-function analyses of the flux observations quantify that a characteristic timescale is identifiable in only one-third of the BL Lacs and that in the majority of the program sources the activity is uncorrelated within the timescales probed. The time-averaged fractional linear polarizations are only on the order of a few percent and are consistent with the presence of tangled magnetic fields within the emitting regions. In many sources a preferred long-term orientation of the electric vector position angle is present. When compared with the very long baseline interferometry structural axis, no preferred position angle difference is identified; the derived distribution resembles that known for core components from very long baseline polarimetry measurements. The polarized flux typically exhibits variability with timescales of months to a few years and exhibits the signature of a propagating shock during several resolved outbursts. The flux and polarization variability indicate that the source emission is predominately due to evolving source components and supports the occurrence of more frequent shock formation in BL Lac parsec-scale flows than in QSO jets, where the magnetic field topology even during outbursts is similar to that of the underlying quiescent flow. The differences that we find in variability behavior and polarization between BL Lacs and QSOs can be explained by differences in stability between the jet flows found by recent studies of relativistic hydrodynamic flows.